1. Field of the Invention
The present invention relates to a sample hold circuit and, more particularly, to a sample hold circuit which reduces the low frequency 1/f noise and also the reset pulse noise in the image signal which is obtained by a charge coupled device image sensor (hereinafter referred to as "CCD" image sensor).
2. Description of the Prior Art
FIG. 1 shows an example of a conventional circuit arrangement of a sample hold circuit for processing a CCD output signal. The circuit comprises a feed-through clamping circuit and a picture signal sample-holding circuit which are connected in series. A typical waveform of an image output signal obtained by the CCD image sensor and processed by such sample hold circuit is as shown in FIG. 2.
As shown in FIG. 2, each picture element of a CCD image output signal can be divided into three periods, namely, those of a reset noise, a feed-through signal and a picture signal. The reset noise is caused by a leakage of a reset pulse signal into the output, the reset pulse signal being for clearing a charge of the preceding picture element in the CCD. The signal required as a picture signal level is a differential voltage A between the feed-through period voltage and the picture signal period voltage in FIG. 2. The reset noise has thus no relation with the picture signal level, and when the picture signal level is small, the signal is buried in the reset noise, resulting in deterioration of the signal to noise (S/N) ratio. Further, usually a low frequency 1/f noise is overlapped or involved in the CCD image sensor output signal and, if this low frequency 1/f noise is not eliminated or removed effectively therefrom, the reproduced image becomes fluctuated or blurred and thus the image quality will be deteriorated.
Therefore, the sample hold circuit for processing the CCD image sensor output signal as shown in FIG. 1 has been proposed for the purpose of eliminating the reset noise and also the low frequency 1/f noise.
First, the CCD output signal inputted from an input terminal 31 is clamped at each picture element, under the feed-through clamping at operation for adjusting the feed-through period voltage of the input signal to a clamping reference voltage V.sub.REF supplied from a terminal 43, by a clamp capacitor 32 and a clamp switch 33, and thus the low frequency 1/f noise is rejected. The feed-through clamping operation is controlled by a clamping pulse C.sub.P externally applied through a terminal 41 to the circuit.
Next, the signal voltage of the largest amplitude level in the picture signal period in FIG. 2 is subjected to a sample-holding operation by a circuit formed by a buffer amplifier 34, a sample hold switch 35 and a hold capacitor 36, and is outputted from an output terminal 38 through a buffer amplifier 37. The sample-holding operation is controlled by a sampling pulse S.sub.P externally applied thorugh a terminal 42 to the circuit. After passing through the overall circuit as described above, the CCD image sensor output signal becomes a picture signal having a level of the feed-through voltage reference, with the low frequency l/f noise as well as the reset noise and feed-through periods having been eliminated.
In the conventional sample hold circuit for the CCD image sensor as described above, where fluctuations occur in the clamping reference voltage V.sub.REF for effecting the feed-through clamping operation, the components of such fluctuations cannot be removed. In the case where these components of fluctuations cannot be removed by the optical black clamping operation which is conducted for each horizontal period by a signal processing circuit at the following stage, the picture quality will be deteriorated, because such components of fluctuations will still appear in the final output signal.
Moreover, since the high speed sampling is executed in the sample hold circuit, the sampling noise or switching noise occurs in transition of switching between the sampling operation and the non-sampling operation. This noise badly influences on the S/N ratio. This sampling noise can be reduced if the band width of the frequency characteristics of the signal route is narrowed towards the low frequency side. However, particularly in the case where a color image sensor is used, since a sample hold circuit for color separation is required in the next stage, the narrowing of the frequency band width, which distorts the wave shape, leads to problems such that the timing control on the sampling hold for color separation becomes much more limited and the control on sampling timing becomes much more difficult, or the color separation cannot be performed accurately depending on the circumstances involved.